With the impending resumption of operations of the LHC, scientists often discuss what they think will be the next big discovery. While it is hard to make predictions, CERN odds-makers are leaning toward a discovery that incorporates supersymmetry, or SUSY, as the clear favorite.
Theories that incorporate SUSY can easily explain why the mass of the Higgs boson is so much lower than one would naturally expect. In fact, it is this aspect of SUSY that has intrigued physicists for decades, leading to more than 10,000 theoretical and experimental papers on the subject. Thus far, in spite of the best efforts by very smart people at both the Fermilab Tevatron and the CERN Large Hadron Collider, no experimental evidence has been found that SUSY is true.
Supersymmetric theories predict a whole class of supersymmetric particles that are cousins to the familiar particle of the Standard Model. In the most common models, these new particles are all unstable, except for the lightest supersymmetric particle (or LSP). From our measurements, we know that the LSP (if it exists) is massive, stable and electrically neutral. (The LSP is actually a leading candidate for dark matter, and this is another reason that SUSY is considered an attractive idea.)
LSPs do not interact very much with ordinary matter and thus will escape any particle collision that produces them without leaving a trace in a particle detector. Using the principle of momentum conservation, we know that the momentum perpendicular to the particle beams must be zero. If we add up the visible momentum and it isn’t zero, we can infer that a particle escaped our detector. Given that LSPs can escape, events with a high momentum imbalance are ideal for searching for SUSY.
The problem is that we have studied events with these characteristics and have seen no evidence for the existence of SUSY. This has led theoretical physicists to invent new theories that predict little or no momentum imbalance. For instance, one such idea postulates that there exist new particles that interact very little with ordinary matter. In this model, the new particle and its supersymmetric cousin have very similar masses. This similarity in mass means that the LSP can have very little momentum, thus no striking momentum imbalance is expected. Theories with this property are generically called stealth SUSY.
CMS scientists studied a class of events with little missing momentum, the signature of W and Z bosons, along with jets from quark production, trying to see if they might find stealth SUSY. No evidence was found. This measurement was used to exclude some parameters in stealth SUSY models.
Source: FNAL, written by Don Lincoln